On Thu, 09 Oct 2008 15:53:31 -0500, "sauwen" <sauwen.jl@gmail.com>
wrote:
>Hi all. Does anyone know how to change the width of a feedback comb
>filter's teeth? From the general idea on wikipedia, it seems like it is
>only possible to change the delays and the gain.
>
>Thanks
Hi,
I'm just thinkin' out loud here, but
perhaps you could cascade the feedback comb
with a feedforward comb.
Both combs must have the same delay. Then
varying the feedforward value (and sign) of
alpha may allow you to "adjust" width of the
cascaded filter's teeth.
[-Rick-]
Reply by Ikaro●October 10, 20082008-10-10
Here is the matlab function that I implemented of the filter on the
article
function [b,a]=reson(fn,fs,K,mthd)
%
% [b,a]=reson(fn,fs,K,mthd)
%
% Generates a resonant (single pole) filter.Parameters are:
%
% fn - desired location of pole (Hz)
% fs - sampling frequency (Hz)
% K - Q factor or pole magnitude(depending on witch method used
% in the mthd parameter described below)
% mthd - (string) desired method to use. Options are:
%
% 'poleMag' : in this case K specifies the magnitude of
the pole in the filter
% (0 < K <1). Notice that the gain might not
be equal to 1 at the resonant
% peak. And this method might provide *less*
attenuaton at the sidebands than
% method using 'Q2' below.
%
% 'Q1' : simple method utilizing a second order
feedback system. K specificies
% the filter's Q (how narrow the filter is).
This method has significant
% drawbacks, use Q2 instead.
%
% 'Q2' : improved version of Q1 with more symetrical
response and unit gain at desired
% pole location (see example).
%
% b - filter FIR taps
% a - filter AR taps
%
%
% %Example
% fs=5000;
% fn=50;
% K=50;
% r=0.997;
% [b1,a1]=reson(fn,fs,K,'Q1');
% [b2,a2]=reson(fn,fs,K,'Q2');
% [b3,a3]=reson(fn,fs,r,'poleMag');
%
% [H1,F]=freqz(b1,a1,500,fs);
% hold on;
% [H2,F]=freqz(b2,a2,500,fs);
% [H3,F]= freqz(b3,a3,500,fs);
% plot(F,20*log10(abs(H1)));hold on;grid on
% plot(F,20*log10(abs(H2)),'r')
% plot(F,20*log10(abs(H3)),'g')
% legend('Q1','Q2','PoleMag')
%Written By Ikaro Silva 2008
switch mthd
case 'poleMag'
cW = cos(2*pi*fn/fs);
b=[1];
a=[1 -2*K*cW K^2];
case 'Q1'
% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Puts a pole exatly on the unit circle
% at the fn Hz line.
%Not recommended!!! Use Q2 instead
cW =2*cos(2*pi*fn/fs);
Hn=[1 -cW 1];
b=[1];
a=[1 K.*Hn];
case 'Q2'
%Similar to 'Q1' but more accurate
%For details see IEEE SP 2008 (5), pg 113
beta=1+K;
f=pi*fn/fs;
numA=tan(pi/4 - f);
denA=sin(2*f)+cos(2*f)*numA;
A=numA/denA;
b=[1 -2*A A.^2];
a=[ (beta + K*(A^2)) -2*A*(beta+K) ((A^2)*beta + K)];
end
%%%End of Function%%%%%
Reply by Ikaro●October 10, 20082008-10-10
Hi,
There was an article that just came out on the IEEE Signal Processing
2008 (5), on the section "DSP Tips& Tricks"
that describes how to design a good variable Q filter (single notch).
hth
On Oct 9, 4:53�pm, "sauwen" <sauwen...@gmail.com> wrote:
> Hi all. �Does anyone know how to change the width of a feedback comb
> filter's teeth? From the general idea on wikipedia, it seems like it is
> only possible to change the delays and the gain.
>
> Thanks
Reply by sauwen●October 9, 20082008-10-09
Hi all. Does anyone know how to change the width of a feedback comb
filter's teeth? From the general idea on wikipedia, it seems like it is
only possible to change the delays and the gain.
Thanks